In the thread about Magazine 72, David Tyler challenged me to improve my ‘2D’ barrel method of making cambered panels into the ‘3D’ method, where broadseams or tucks have been added. I stated that I could make broadseams. That is only half true (..me and my big mouth...). I have designed and made them, but that was for a staysail! The problem is not the sewing bit, but rather how to calculate the number and widths of those broadseams in a junksail..
Take a look at the photo of Johanna to the left. Johanna’s sail was assembled from a number of flat barrel-shaped panels. After sailing it for a couple of trips, I found that there was less camber than my Chain Calculator had predicted. Then I remembered that I had pulled the sail fairly taut along each batten. To cure it, I slackened the sail 100mm at the luff of each batten. The measured camber then increased to the calculated 8%, turning the sail into a real muscle sail.
The bi-product of that adjustment was a row of wrinkles along the battens (..very visible in the low sun..).
The other photo shows me measuring the actual camber on Frøken Sørensen’s sail. On this sail I aimed for 9% camber. When rigging this sail, I pulled it just a bit taut along the battens, less than on Johanna, I think. The sail then ended up with about 8% camber, which I thought looked good and worked well.
Then have a look at a part of the sail plan for my present boat, Ingeborg, to the right. Her sail is already rigged, but I have yet to measure her actual camber (I aimed for 8%). On the plan I have measured the length of a curve compared to the batten length. As can be seen, on the imaginary 8% camber curve at the bottom of the diagram, the curve is 87mm longer than the 4.9m batten. In practice this cannot happen, so to make the panel bulge enough, the tension has to be eased a bit along the battens.
· The slackness can either be left visible, as on Johanna’s sail (wrinkles),
· one can tauten the sail, just a little to reduce the wrinkles, at the cost of a bit camber,
· or one can shorten the sail along the battens using 3(?) broadseams.
What confuses me is how much of those extra “curve lengths” in the diagram should be taken up by the broadseams. Moreover, what worries me is that I can overdo it. Then I lose my secret little way of limiting the camber a bit by just stretching it along the battens.
I know Slieve has made a spreadsheet about the matter, but I kind of grey out before getting out a useful result.
Any suggestions are welcome.
Arne
